This invention relates to web drive means and more particularly to a means for controlling the wrap angle and tension of a web over a drive roller.
In various apparatus, web surfaces are employed to either carry a separate substrate or a coating on the web which is to be acted upon within the apparatus. In most instances, drive rollers are employed to provide the motive force for pulling the web through the apparatus. In order to achieve the proper tension over the drive roller and thereby the proper amount of friction, a capstan is normally employed. In many instances, a capstan has been found to be undesirable either because of the lack of an ability to adjust continuously the tension on the drive roller or the coating on the web will not withstand the contact required with the capstan device.
There has been devised various mechanisms for controlling the wrap angle and tension of a web over a roller for purposes other than to provide friction for driving the web. For example, a web positioning apparatus is disclosed in U.S. Pat. No. 3,172,187 which comprises a pair of rollers on arms extended between a pair of drive rolls, each roller being urged against the web. As the pressure arms are pivoted away from each other, the wrap angle of the web around one of the drive rolls decreases until no contact at all is permitted between the web and one of the rolls.
A similar device is described in U.S. Pat. No. 3,591,151 wherein a pair of arms carrying rollers is positioned on a fulcrum within the circumference of a heated drying roll. The web is threaded over the rollers on the arms at the entry and exit points from the roll. By pivoting the arms, the amount of wrap around the roll is controlled in accordance with the circumference distance between the rollers on each arm. In this way, a web may be dried by conducting the web over the hot roll for a distance to be determined variably by the position of a pair of rollers at the circumference of the drying roller. Similarily, an apparatus for uniformly heating a band shaped material is disclosed in U.S. Pat. No. 2,972,668 wherein a pair of arms are extended on each side of a drying roll over which a band or web is threaded such that the amount of contact with the drying roll is controlled by extending the distance between the two arms in an arcuate path thereby withdrawing the band from contact with the roll.
While the prior art devices accomplish the objectives variously set forth, there are some processes employing web members wherein a constant drive means is essential depending upon different thicknesses of the webs and the kind of material carried by the webs. Accordingly, there is now provided an improved web forwarding means which avoids the disadvantages found in the prior art.